1. Field of the Invention
The present invention relates to a recirculating document feeding apparatus and, more particularly, it relates to a recirculating document feeding apparatus used with a copying machine, an image reader and the like by which sheet originals are recirculated from a tray through an exposure position to the tray.
2. Description of the Prior Art
Conventionally, for apparatuses such as a copying machine, an image reader, a facsimile, and the like, there are three methods of slit exposure. In the first method, which is the most popular method, an original is placed on a fixed original support table to be mirror scanned. In the second method, which is widely used in inexpensive machines, the original support table is moved to scan the original placed thereon. In the third method, originals are automatically fed successively to the exposure position, and the moving originals are copied.
Few apparatuses employ the third method because of the following reasons.
(1) The originals are liable to be damaged, since they are handled much more than in other methods.
(2) A book or a thick original can not or hardly be handled, since the original must be transferred.
The advantage of the third method is that a large number of sheet originals can be successively fed with a constant interval and the originals can be processed at high speed since the method requires no return time after the scanning, which time was required in the method of mirror scanning or in the method in which the original support table is moved.
As an example of a document feeding apparatus employing the third method, a so-called recirculating document feeding apparatus is proposed in U.S. Pat. No. 4,169,674, in which sheet originals placed on a tray are successively fed to the exposure position and, thereafter, returned from the exposure position to the tray again. In such recirculating document feeding apparatus, the first disadvantage can be eliminated, since the reliability of feeding paper to the machine is enhanced and, by providing a detecting mechanism properly, the reliability can be still further enhanced. However, the second disadvantage has not yet been completely eliminated.
In another type of original feeding apparatus such as shown in Japanese Examined Patent Publication No. 30621/1980, a glass support used only for a sheet original and another glass support used only for a thick original are provided. In an apparatus shown in Japanese Examined Patent Publication No. 42462/1985, an automatic document feeder for sheet originals is separately arranged on a side of the fixed original support table for placing a thick original, and original glass supports are respectively provided thereon. In this apparatus, the switching control between the mirror scanning for the original support table and the automatic document feeding is carried out. These conventional original automatic feeding apparatuses by which the thick original can be fed must be enlarged and complicated by providing the original glass supports respectively for the sheet original and thick original.